The invention relates to a drive-slip control device (ASR) for a road vehicle brake system equipped with an anti-lock system (ABS) and having a hydraulic dual-circuit brake system. One brake circuit is assigned to the driven vehicle wheels and the other brake circuit to the non-driven vehicle wheels and the two brake circuits are designed as static brake circuits, each of which is assigned to an outlet-pressure space of a brake unit provided for brake actuation. The brake system operates in accordance with the following:
At least on the brake circuit of the driven vehicle wheels, the ABS works on the return-flow principle during pressure-reduction phases of the anti-lock control, to return a quantity of brake fluid (corresponding to that quantity of brake fluid flowing off from the wheel brake(s) undergoing the control) into a return line connected to a buffer accumulator for pumping back into the brake unit by a return pump which has its inlet side connected to the buffer accumulator and its outlet side to the brake unit;
Wherein the ASR works on the principle of decelerating a vehicle wheel tending to spin by subjecting its wheel brake to pressure from an auxiliary-pressure source such that its drive slip remains within a value range compatible with a sufficient dynamic stability of the vehicle;
Wherein during drive-slip control mode, the return pump of the ABS assigned to the brake circuit of the driven vehicle wheels is utilized as an auxiliary-pressure source from which pressure is supplied to the wheel brake(s) in pressure build-up phases;
Wherein there is an ASR control valve which can be changed over from a basic normal braking mode position in which brake fluid can be positively displaced into the wheel-brake cylinders of the brakes of the driven vehicle wheels as a result of the actuation of the brake unit, into an alternative functional position, in which brake fluid is prevented from flowing off from the main brake line of the brake circuit of the driven vehicle wheels towards the respective outlet-pressure space of the brake unit.
An ASR of this general type is the subject of the applicant's own older published Patent Application U.S. Ser. No. P 3802 133.1-21 which describes (in combination with an ABS working on the known return-flow principle), an ASR which works on the principle of decelerating a driven vehicle wheel tending to spin by subjecting its wheel brake to pressure. There the return pump of the ABS assigned to the brake circuit of the driven vehicle wheels is utilized as an auxiliary-pressure source for the ASR. This return pump is conventionally designed as a free-piston pump and is fed via a precharging pump in the ASR control mode. The precharging pump feeds brake fluid from the brake-fluid reservoir of the brake system to the return line of the ABS via an outlet non-return valve generating brake pressure during the ASR mode. The return pump conveys brake fluid into the main brake line which is shut-off from the brake unit by means of an ASR control valve during drive-slip control. Furthermore, there is an ASR outlet valve which is connected between the return line of the brake circuit of the driven vehicle wheels and the brake-fluid reservoir. The basic position of this ASR outlet valve is its shut-off position and it is moved into its throughflow position only in pressure-reduction phases of the ASR to allow brake fluid to flow off from the return line towards the brake-fluid reservoir.
This return line is pressureless in the normal braking mode (braking not undergoing a brake-pressure control). Consequently, a leak in the admission-pressure supply path, leading via the return line of the return pump utilized for the drive-slip control (for example a leak of the outlet non-return valve of the precharging pump or a leak of the ASR outlet valve) cannot be detected in the normal braking mode.
However, if such a leak is present and the front-axle brake circuit fails during a braking operation, then when the ABS acts on the rear-axle driven wheel brake circuit during this braking operation the brake circuit can empty via the leak. Hence both brake circuits will fail. This complete failure of the brake system remains permanent. This potential danger that a failure of the front-axle brake circuit will cause the rear-axle brake circuit to also fail, if there is a leak in the return-pump supply circuit, is naturally unacceptable.
This problem does not arise in a brake system known with an anti-lock and drive slip control from DE 3,627,809 Al. There the ABS works on a return-flow principle modified to the effect that as soon as the anti-lock control responds, the return pumps assigned to the individual brake circuits not only pumps the quantity of brake fluid responding to that bled from the wheel brakes back into the outlet-pressure spaces of the master cylinder, but additionally pumps such a quantity of brake fluid out of the reservoir into outlet-pressure spaces of the brake unit so that the master-cylinder pistons are pushed back to their basic positions which correspond to the non-actuated state of the brake system. There central valves integrated in the master-cylinder pistons then act as brake valves, via which the brake circuits are now opened towards the reservoir to such an extent that the dynamically established brake pressure caused by the opening state of these valves and the delivery capacity of the pumps is proportional to the pedal force with which the driver actuates the brake pedal. This prevents the supply of brake fluid in the master-cylinder outlet pressure space (of the particular brake circuit undergoing anti-lock control) from being exhausted with a consequent failure of the brake circuit. However, there is disadvantage in that whenever the anti-lock control responds, a very uncomfortable pedal reaction occurs. In particular, a sudden jolt-like pushing of the brake pedal back to its initial position at the onset of the control occurs and as a result thereof, a hard brake pedal feeling lasts as long as the control lasts. Additional special construction of the central valves also has to be provided, since in this further known brake system, these valves must be capable of opening reliably within a short travel under extreme conditions corresponding to the maximum selectable brake pressure. A value design suitable for this purpose requires that the valve bodies and valve seats of the central valves have smooth metallic sealing faces, in contrast to conventional central valves having elastomeric sealing faces on the seats and/or the valve bodies. This of course involves considerable additional outlay in terms of both manufacturing accuracy and the need for additional filter elements to ensure that the sealing metal faces cannot be soiled. This is true since with valve having metal sealing faces (both on the seat and on the valve body) even the smallest dirt particles are sufficient to prevent a sealed closing.
The object of the instant invention is to therefore provide a comparatively simply designed ASR of the type mentioned and to the effect that even when a leak is present in the inlet circuit of the return pump of the brake circuit of the driven vehicle wheels (which is utilized as a pressure source for the drive-slip control) and when the brake circuit of the non-driven vehicle wheels fails during a braking operation, there is nevertheless sufficient safety against a failure of the brake circuit of the driven vehicle wheels.
According to the invention, this object is achieved by having a supply flow path lead from a brake-fluid reservoir of the brake unit through a piston of the brake unit and a delivery outlet of the pressure space of the brake unit to the inlet of the return pump. An inflow control valve is connected between the delivery outlet and the inlet of the return pump of the brake circuit of the driven vehicle wheels and is changed over from its shut-off basic position assigned to the braking mode to its throughflow position assigned to the ASR mode for the duration of control cycles of the drive-slip control.
Because the supply flow bath (by which brake fluid can be fed to the return pump from the brake-fluid reservoir of the brake system) has a compensating flow path which is open in the non-actuated state of the brake unit to an outlet-pressure space of the brake unit, and because it can be shut-off by means of an inflow control valve which is connected between the outlet of the brake unit and the return pump during the braking mode, and because it is only switched to its throughflow position in the drive-slip control mode, this will prevent the possibility that the brake will empty towards the brake-fluid reservoir (in the event of a response of the anti-lock control on the brake circuit of the driven vehicle wheels). This is true since there is no flow path which leads directly back to the brake-fluid reservoir of the brake system. If, on the other hand, the inflow control valve leaks during normal braking, this can be detected from a lengthening of the pedal travel, since even during normal braking, the buffer accumulator of the rear-axle brake circuit has to be filled before there can be an appreciable brake-pressure build-up in the brake circuit of the driven vehicle wheels. Thus, in practice, this guarantees increased safety because a leak of the inflow control valve is detectable.
With a design of the inflow control valve as an electrically activated 2/2-way solenoid valve, this valve can be actuated at the same time as the activation of the precharging pump and of the return pump of the brake circuit of the driven vehicle wheels in response to an output signal from the electronic ABS and ASR control unit.
Instead of an electrically activatable inflow control valve, one can use a mechanically controlled valve which can be actuated directly by means of the brake pedal to be changed over from its throughflow drive-slip control basic position to its shut-off functional position assigned to the braking mode.
This change over can occur if displacement of the piston (which movably limits that outlet-pressure space of the brake unit assigned to the brake circuit of the driven vehicle wheels) actuates the valve and if this change-over takes place within a small fraction of the possible total stroke of the brake-unit piston, it affords a constructively favorable possibility of integrating the inflow control valve into the brake unit. A substantial advantage in replacing a solenoid valve with a mechanically activatable valve is to be seen in the essentially simpler design of such a valve and also in a price benefit resulting from this change.
If two electrically actuated 2/2-way solenoid outlet valves are inserted into the return line of the hydraulic unit of the ABS and ASR to control brake fluid flow-off from one or both wheel brakes of the brake circuit of the driven vehicle wheels, during pressure-reduction phases of the anti-lock control, an individual anti-lock control on the two driven vehicle wheels is possible. Hence brake pressure can be reduced on one wheel brake and brake pressure built up on the other wheel brake. Also a coupled drive-slip control is possible, when brake pressure is built up on one wheel brake and in the other wheel the brake pressure is at most maintained, but cannot be reduced.
The manner of inserting the outlet valves of the wheel brakes of the driven vehicle wheels is provided by having one outlet valve of the two outlet valves shut-off the wheel brake of one the driven vehicle wheels from the another and from the return line of their brake circuit during drive-slip control. This valve can thus connect the wheel brakes to one another at least in the anti-lock control mode. The other outlet valve is position to jointly shut-off or connect the return line to both wheel brakes.
However, it is possible, to use a hydraulically activatable outlet control valve, which is activatable hydraulically by means of the pressure built up during the actuation of the brake unit in the secondary outlet pressure space assigned to the brake circuit of the driven vehicle wheels, and which is changed over from its shut-off basic position into its throughflow position by means of this pressure. This construction is suitable for a common anti-lock control of the two wheel brakes, in which brake pressure is built up, held, or reduced in phase on both vehicle wheels. The drive-slip control is coupled in such a way that whenever the brake pressure on one of the wheel brakes of the driven vehicle wheels is changed, the brake pressure on the other wheel can either be changed in the same direction or be maintained.
However, it is possible, to use a hydraulically activatable outlet control valve, which is activatable hydraulically by means of the pressure built up during the actuation of the brake unit in the secondary outlet pressure space assigned to the brake circuit of the driven vehicle wheels and which is changed over from its shut-off basic position into its throughflow position by means of this pressure instead of solenoid actuated outlet valve to make the connection between the wheel brakes. This construction markedly reduces the technical outlay required for the ASR control.
One could alternatively have the outlet valve designed as a mechanically activatable 2/2-way valve to make the connection between the wheel brakes of the brake circuit of the driven vehicle wheels. This valve can be changed over from the shut-off basic drive-slip control position into throughflow braking bode position as a result of the pressure build-up displacement occurring whenever the brake unit is actuated. Here the brake-unit piston which movably limits the secondary outlet-pressure space of the brake unit assigned to the brake circuit of the driven vehicle wheels mechanically actuates the outlet control valve with the change-over taking place within a small fraction of the possible total stroke of the brake-unit piston.
Alternatively to this, the outlet pressure of the precharging pump could cause activation of the outlet valve making the connection between the wheel brakes. Here the basic position is the throughflow position and the functional position assigned to the drive-slip control mode is the shut-off position.
The precharging pump could be a gear pump to convey brake fluid out of the brake-fluid reservoir of the brake system to the pump chamber of the return pump of the brake circuit of the driven vehicle wheels. Since the leakage rate at standstill is very high, a control slide, activated by means of the outlet pressure of the precharging pump, is provided for actuating the outlet valve and the pressure relieved sufficiently quickly so that the ASR can respond quickly on a wheel brake otherwise shut-off.
A marked simplification of the hydraulic unit of the ASR is achieved by means of a precharging pump which is constructed as a further piston pump which can be driven by means of the pump drive provided for the return pump. Utilizing the design with a self-priming pump is advantageous if the precharging pump is equipped with a displacement controlled inlet valve which is moved into its open position while the pump piston is executing the final portion of its suction stroke and the initial portion of its delivery stroke and which is otherwise shut-off. This results in an especially low inlet flow resistance.
This is also true when the return pump of the brake circuit of the driven vehicle wheels is designed as a self-priming pump, since then there is no need for a precharging pump for the return pump to act as an auxiliary-pressure source of the ASR.
Alternatively to an insertion of the ASR control valve between the brake unit and the main brake line of the brake circuit of the driven vehicle wheels, it is also possible to insert the ASR control valve into the ABS and ASR hydraulic unit between the brake-fluid reservoir and the brake unit.
It is advantageous if fault detection device is provided to generate an indicator signal characteristic of the proper functioning or malfunctioning of the brake system as a result of a comparing a first sensor output signal (which is a measure of the displacement of the piston of a brake unit occurring as a result of an actuation of the brake system) with a second sensor output signal (which is a measure of the brake-actuating force arising as a result of the actuation). The second sensor output signal is generated by a pressure sensor which monitors the brake pressure occurring in the brake circuit of the driven vehicle wheels. If the brake unit is designed as a tandem master cylinder, then the fault detection device could comprise two displacement sensors, each monitoring the position of one of the two pistons of the tandem master cylinder and generating electrical output signals characteristic of the piston positions. Comparison of these signals would generate indicator or warning signals characteristic of the proper functioning or malfunctioning of the brake system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.